Highly contoured, multi-leg composite structures may be used in a variety of applications, such as in aerospace vehicles. Aerospace vehicle frames, spars, ribs, stringers and similar structures may have multiple legs which are contoured along the length of the structure. One known technique for fabricating such structures involves hot drape forming a flat composite charge over multiple contoured tools in order to form one or more curved legs in the structure. A charge typically comprises multiple plies of unidirectional prepreg fiber laid up according to a predetermined ply schedule. Difficulty may be encountered however, when attempting to hot drape form flat charges that contain zero degree plies, i.e. plies having a fiber orientation that is aligned substantially parallel with the axis of curvature of the structure. This difficulty stems from the fact that where zero degree fibers are used that are substantially inextensible, such as carbon fibers, the fibers may not undergo a change in geometry, i.e. bunching, stretching or spreading during the forming process. Consequently, zero degree fibers may not be moved from one plane to a different plane that possesses a substantially different arc length. If the leg is too convex, the fibers may wrinkle, whereas if the leg is too concave, the fibers may lock in place and resist out-of-plane forming pressures.
To overcome the above-mentioned problem, zero degree plies are hand-placed on the charge after the non-zero plies have been formed into curved legs. The need to place the zero degree plies on a leg after it has been drape formed must be repeated each time a zero degree ply is required by the layup schedule. These multiple operations increase costs, complicate tooling, may add to floor space requirements and may adversely affect manufacturing flow times.
Another disadvantage of known techniques for forming highly contoured composite structures having multiple legs relates to the need for forming each of the legs in a separate forming operation using different sets of tools. Forming the legs of the structure using different sets of tools also may contribute to higher costs and longer production flow times.
Accordingly, there is a need for a method and apparatus for producing contoured composite structures having one or more legs that reduce the number of operations required to form the legs, while avoiding bunching or wrinkling of the zero degree plies during forming. There is also a need for apparatus for forming such structures that allows forming two curved legs on the structure using a single tool in a single forming operation.